Ligaments compromise the fibrous connective tissues, which, in combination with joint geometry, are responsible for limiting the range of motion and for transmitting forces and moments across all skeletal joints of the body. One of the skeletal joints whose ligaments are susceptible to injury which often leads to chronic ligament deficiency is the knee joint. Knee orthoses or braces have been commercially available for some time to prevent abnormal motions and thus protect knee ligaments from being excessively stretched as they heal following injury or following surgical repair or reconstruction, to brace knees which are chronically unstable due to ligament deficiency and to prevent injury to the knee in the event of a fall or during athletic activities. Knee braces generally comprise a femoral cuff adapted for attachment to the thigh, a tibial cuff adapted for attachment to the calf and linkage which interconnects the femoral and tibial cuffs to permit pivotal movement therebetween.
Although often modeled as a hinge joint, the knee is in fact a polycentric joint whose motions are a combination of rolling and sliding movement between the contacting tibial and femoral condyle surfaces. In order to duplicate the complex anatomical motion of the knee, a knee brace must replicate the translational and rotational movements of the knee in three dimensions, i.e., the brace must replicate six component motions or "degrees of freedom". This task has proven to be virtually impossible to accomplish due to differences in the knee of one individual compared to another, and due to the difficulty in maintaining the knee brace at the proper position relative to the knee axis as that axis shifts position as the knee undergoes flexion-extension and internal-external rotation.
Prior art knee braces have been designed to locate the brace axis, i.e., the pivot axis of the linkage interconnecting the femoral and tibial cuffs, substantially coaxial with the flexion-extension axis of the knee with the lower leg in one position, or throughout partial movement of the leg. But the brace axis of such prior art knee braces is not maintained substantially coaxial with the knee axis throughout the complete range of normal translational and rotational movement of the knee. Nevertheless, the suspension system which mounts such prior art braces to the leg is designed to maintain the brace, and, in turn, the brace hinging axis, in an essentially fixed position on the leg. As a result, the brace fails to replicate normal movement of the knee joint and applies forces to the knee which are a combination of forces in the anterior-posterior direction and forces generally parallel to the longitudinal axis of the tibia. The resultant of these forces can interfere with the normal knee motion, create unnatural stresses on the knee, cause a loss of desired control of knee motion and induce a "pistoning" or shearing force between the cuffs of the brace and the skin. This pistoning or shearing force is believed to be a contributing factor to distal migration of the brace in which the brace tends to move toward the lower leg from its initial position. These problems contribute to wearer discomfort, skin irritation and possible damage to the knee joint.
Prior art braces are predominantly intended to control abnormal translation or rotation through the entire range of motion of the knee, but at least some braces have incorporated elements directed particularly to controlling selected knee motions for the protection of specific ligaments or ligament groups. For example, prior art knee braces are commercially available which incorporate a four-point suspension system for mounting the femoral cuff to the thigh and the tibial cuff to the calf. These braces are intended to control anterior or posterior translation of the tibia with respect to the femur for the protection of either the anterior or posterior cruciate ligaments.
Prior art four-point suspension systems for anterior cruciate ligament deficiencies, for example, employ four separate straps including a strap wrapped around the femoral cuff at the proximal-most part of the brace, a relatively inelastic strap wrapped posteriorly about the thigh immediately proximal to the patella, a strap wrapped anteriorly about the calf at the level of the proximal portion of the tibial cuff and a strap wrapped about the calf at the distal-most part of the tibial cuff of the brace. In response to an anterior force applied to the tibia by walking, climbing stairs or other movement, this arrangement of straps is intended to apply a restraining force to the tibia restricting its anterior translation relative to the femur. Knee braces for posterior cruciate ligament deficiencies have a similar construction except that the anterior and posterior straps are reversed.
One problem with the four-point suspension systems of prior art knee braces is that none account for the initial movement of the soft tissue when the brace is first loaded in response to an anterior or posterior force on the tibia. Restraining forces applied by the brace to the tibia must be transmitted through the soft tissue of the leg. Initially, the stiffness of the soft tissue is low and it compresses or deforms under load. Thus a relative motion occurs between the brace and the underlying bone whose motions are to be controlled. The restraining force provided by the brace depends upon amount of bone motion or soft tissue compression, the stiffness of the soft tissue and the brace design. As the tissue progressively deforms, its stiffness increases and so does the restraining force. Unfortunately, by the time soft tissue has been compressed sufficiently to effectively transmit forces from the brace to the tibia, unwanted translation of the tibia has occurred. As a result, prior art knee braces of this type are often ineffective in preventing abnormal anterior-posterior movement of the tibia and thus fail to protect the knee against such abnormal movement.